There is a desire to implement power amplifier (PA) circuits for radio frequency (RF) transmitters using CMOS technology having high efficiency and wide dynamic range for both RF power control and complex modulation schemes, such as OFDM and PSK. Ideally, such power amplifiers provide the capacity to generate a signal which includes amplitude modulation having more than 20 dB depth, for OFDM applications, and have the capability to provide over 70 dB of transmit power control, for CDMA applications.
GaAs and other group III-V materials have yet to be integrated into a high density CMOS process (0.18 um) so a single chip solution is not yet available. State of the art CMOS linear power amplifiers do not report high efficiency, primarily due to the high transistor saturation voltage. Thus, this precludes these PAs from being accepted in the market on the grounds they provide significantly reduced battery life for portable devices.
On the other hand, CMOS switched PAs, such as those available from Silicon Laboratories Inc., only operate with constant amplitude envelope signals. Unfortunately, these PAs do not have power control of sufficient range for CDMA applications. Power control for these PAs is achieved by reducing the drain and source terminal voltage (Vds) for the FET used therein. This reduction in Vds provides approximately 30 dB of power control, whereas CDMA and WCDMA applications require in excess of 60 dB power control.
CMOS PAs offer reasonable operating efficiencies when operated in a switched mode of operation, unfortunately, these switched mode PAs are only acceptable for use with RF signals that have a constant amplitude envelope. When CMOS PAs are extended to operate with RF signals having amplitude modulation, they are significantly less efficient than bipolar silicon amplifiers or group III-V material amplifier equivalents. This drop in efficiency is attributable to the high ratio of saturation voltage to maximum operating voltage.
A need therefore exists to provide a PA circuit architecture that uses a CMOS process in a switched amplifier configuration but that is capable of providing an amplitude modulated output signal. In order to provide the amplitude modulated output signal modulation, typically an input signal is split into two or more signals that are thereafter amplified and then combined. Relative phasing of these amplified signals determines both the phase and magnitude of the combined output signal. This type of amplification process is known to those of skill in the art as linear amplification using nonlinear components, or LINC. A LINC amplifier of a radio frequency transmitter provides substantially linear amplification by using two nonlinear amplifier circuits and by decomposing the input signal into two constant amplitude envelope, phase varying signals, which, when combined, constructively and destructively interfere to re-form an amplified version of the original signal
For CDMA and WCDMA systems, the power control range is substantially larger than the AM depth required for the modulation, approximately 70 dB and 12 dB, respectively. A LINC type architecture by itself cannot provide gain and phase matching to achieve in excess of 80 dB dynamic range.
It is therefore an object of the invention to provide a PA circuit architecture that is manufactured within a CMOS process and operates using a switched amplifier configuration and offers a large dynamic range.